class output_command :
?onclose:unit -> unit -> string ->
/bin/sh, and data written to the channel is piped to stdin of the command.
pos_out returns the number of written octets.
close_out is invoked, the subprocess is
waited for. If the
process exits with code 0, the method returns normally. Otherwise,
Command_failure is raised. (The channel is closed
even if this exception is raised.)
onclose: this function is called when the
close_outmethod is invoked, just after the underlying descriptor has been closed.